Coatings are applied in the manufacture and finishing of a vast range of goods and products including metal sheet and coil, timber flooring, automobiles, aeroplanes, fencing, glass, packaging and the like. In the area of metal sheet and coil, the substrate is typically galvanized steel, aluminium, stainless steel, or zinc alloy coated steels including aluminium-zinc and zinc-magnesium coated steels.
Devices are available to measure the coating thickness on these products to avoid costly wastage of the coating material, and to ensure the quality of the finished product. These devices include magnetic sensors, ultrasonic sensors, and mechanical-optical devices such as the commercially available DJH gauge system (DJH Designs Inc. Oakville, ON, Canada). This system involves mechanically boring a shallow crater through a painted surface into the underlying substrate of a sample. The sample is then placed under a microscope and the crater is viewed on a high resolution monitor for determination of the coating thickness. Likewise, coating thickness can be determined by sectioning a sample cut from a coated product and viewing the sample under high magnification. Other methods include the use of a micrometer to measure thickness of the coated article, stripping off the coating, re-measuring the article, and calculating the coating thickness as the difference between the two readings. However, these techniques either have limited accuracy, or they are time consuming and labour intensive. Moreover, they do not measure or monitor the thickness of the coating during the coating process.
Lasers have also been used in the past for the measurement of coating thickness, for example, as described U.S. Pat. No. 848,785 (EP 2 031 347 A1). This involves the use of a laser sensor to measure the size of the substrate before it is coated, and to monitor the coating thickness during the application/deposition of the coating to determine the end point for the coating deposition process. Similarly, US 2005/0137829 describes a thickness measurement system that utilises a laser displacement sensor which measures the distance to the surface of the substrate before and after the substrate is coated with a coating.
U.S. Pat. No. 5,757,498 describes an optical spray coating monitoring system employing a laser displacement sensor and a data acquisition system for monitoring the spray gun operator's coating application technique and determining the coating thickness. The system includes 2 lasers mounted on the spray gun itself, one of which is arranged to measure the distance to the substrate and the other of which measures the distance to the surface of the wet coating as the coating is being applied to the substrate.
U.S. Pat. No. 6,092,419 describes a system for measuring coating thickness which employs a short pulse laser for generating an ultrasound signal in a wet or dry paint film on a substrate, and a detection laser for detecting minute motions of the surface of the paint film arising from the ultrasound signal induced in the film. An interferometer is used to record modulations in the laser light pulse frequency of the detection laser for determination of the film thickness, the resonance of the paint film being directly related to its thickness.
U.S. Pat. No. 6,120,833 describes a system for measuring coating thickness utilising signals from an inductive sensor and a capacitive sensor to measure the thickness of a coating on a substrate travelling over support rolls.
U.S. Pat. No. 5,409,732 describes a method for regulating the thickness of a wet coating applied to a web substrate. This system utilises optical density sensors to measure periodic variations in the thickness of the coating on the web. An automatic controller analyses the information from the sensors and adjusts the gap between metering bar and the substrate surface to remove excess coating applied to the web substrate. Alternatives to optical density sensors are described as including beta gauge and capacitive gauge sensors, or a physical measurement of the combined web and coating thickness.